1. Field of the Invention
The present invention relates to throttle control devices for controlling a flow rate of intake air supplied to an engine, e.g., an internal combustion engine of an automobile, and in particular to throttle control devices that are electrically or electronically controlled.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2001-59702 teaches a known throttle control device that includes a throttle valve disposed within an intake air channel formed in a throttle body. The throttle valve is rotatably driven by a motor in order to open and close the intake air channel, so that the flow rate of the intake air is controlled. The throttle control device further includes a throttle sensor (also known as “throttle position sensor”) that detects the degree of opening of the throttle valve. The throttle sensor includes a pair of magnets and a magnetic detecting element, such as a Hall element. The magnets are attached to a support member. The support member is mounted to at throttle shaft that rotates in unison with the throttle valve, so the magnets are positioned to oppose to each other with respect to the rotational axis of the support member. The magnetic detecting element is mounted to the throttle body. The magnetic detecting element detects the intensity of the magnetic field produced by the magnets and outputs the detected intensity as signals that represent the degree of opening of the throttle valve.
However, because the magnetic detecting element detects the intensity of the magnetic field produced by the pair of magnets, the magnetic detection element may output incorrect signals if the pair of magnets has been offset from their initially set positions relative to the magnetic detection element. The offset could be due to possible displacement of the throttle shaft during a long period of use or due to thermal expansion of the molded resin that incorporates the magnets through an insert molding process. Such incorrect signals also may be outputted if the level of intensity of the magnetic field has been changed due to temperature-dependent characteristics of the magnets. For these reasons among others, the detection accuracy of the degree of opening of the throttle valve may be lowered, and therefore, the accuracy of the control of the flow rate of the intake air may also subsequently be lowered. This problem becomes more significant if the throttle body is made of a synthetic resin that has a large coefficient of thermal expansion or if the throttle body is made of a material that cannot be accurately formed or machined. Therefore, it has been desired to improve the known throttle control devices and reduce these problems.
To this end, Japanese Laid-Open Patent Publication No. 8-35809 has proposed a device 101a for detecting a rotational angle, as shown in FIG. 6 of the publication, in which a pair of stators 160 and 161, each having a semi-circular cross section, are disposed within the yoke 110. A gap 162 is formed between the stators 160 and 161. The sensor 170 is positioned within the gap 162 for detecting the strength of a magnetic field. With this arrangement, the direction of the magnetic field is directed in primarily one direction, i.e., a direction indicated by the arrows shown across the gap 162 as viewed in FIG. 6 of the publication. The magnetic field is most unidirectional particularly where the magnetic field lines intersect the sensor 170, throughout a change in the rotational angle of the yoke 110. Therefore, the sensor 170 can properly detect the rotational angle of the rotary shaft over the entire range of rotation.
However, incorporation of the stators 160 and 161 may increase the total number of parts required for a device used in detecting rotational angles and therefore may increase the overall manufacturing cost. In addition, an increase in the number of parts may consequently demand increased accuracy in the assembling operation.